Thermal transfer printing has displaced impact printing in many applications due to advantages such as the relatively low noise levels which are attained during the printing operation. Thermal transfer printing is widely used in special applications such as in the printing of machine readable bar codes and magnetic alpha-numeric characters. The thermal transfer process provides great flexibility in generating images and allows for broad variations in style, size and color of the printed image. Representative documentation in the area of thermal transfer printing includes the following patents.
U.S. Pat. No. 3,663,278, issued to J. H. Blose et al. on May 16, 1972, discloses a thermal transfer medium comprising a base with a coating comprising of cellulose polymer, thermoplastic aminotriazine-sulfonamide-aldehyde resin, plasticizer and a "sensible" material such as a dye or pigment.
U.S. Pat. No. 4,315,643, issued to Y. Tokunaga et al. on Feb. 16, 1982, discloses a thermal transfer element comprising a foundation, a color developing layer and a hot melt ink layer. The ink layer includes heat conductive material and a solid wax as a binder material.
U.S. Pat. No. 4,403,224, issued to R. C. Winowski on Sep. 6, 1983, discloses a surface recording layer comprising a resin binder, a pigment dispersed in the binder, and a smudge inhibitor incorporated into and dispersed throughout the surface recording layer, or applied to the surface recording layer as a separate coating.
U.S. Pat. No. 4,463,034, issued to Y. Tokumaga et al. on Jul. 31, 1984, discloses a heat-sensitive magnetic transfer element having a hot melt or a solvent coating.
U.S. Pat. No. 4,628,000, issued to S. G. Talvalkar et al. on Dec. 9, 1986, discloses a thermal transfer formulation that includes an adhesive-plasticizer or sucrose benzoate transfer agent and a coloring material or pigment.
U.S. Pat. No. 4,687,701, issued to K. Knirsch et al. on Aug. 18, 1987, discloses a heat sensitive inked element using a blend of thermoplastic resins and waxes.
U.S. Pat. No. 4,707,395, issued to S. Ueyama et al., on Nov. 17, 1987, discloses a substrate, a heat-sensitive releasing layer, a coloring agent layer, and a heat-sensitive cohesive layer.
U.S. Pat. No. 4,777,079, issued to M. Nagamoto et al. on Oct. 11, 1988, discloses an image transfer type thermosensitive recording medium using thermosoftening resins and a coloring agent.
U.S. Pat. No. 4,778,729, issued to A. Mizobuchi on Oct. 18, 1988, discloses a heat transfer sheet comprising a hot melt ink layer on one surface of a film and a filling layer laminated on the ink layer.
U.S. Pat. No. 4,923,749, issued to Talvalkar on May 8, 1990, discloses a thermal transfer ribbon which comprises two layers, a thermosensitive layer and a protective layer, both of which are water based.
U.S. Pat. No. 4,975,332, issued to Shini et al. on Dec. 4, 1990, discloses a recording medium for transfer printing comprising a base film, an adhesives improving layer, an electrically resistant layer and a heat sensitive transfer ink layer.
U.S. Pat. No. 4,983,446, issued to Taniguchi et al. on Jan. 8, 1991, describes a thermal image transfer recording medium which comprises as a main component, a saturated linear polyester resin.
U.S. Pat. No. 4,988,563, issued to Wehr on Jan. 29, 1991, discloses a thermal transfer ribbon having a thermal sensitive coating and a protective coating. The protective coating is a wax-copolymer mixture which reduces ribbon offset.
U.S. Pat. Nos. 5,128,308 and 5,248,652, issued to Talvalkar, each disclose a thermal transfer ribbon having a reactive dye which generates color when exposed to heat from a thermal transfer printer.
And, U.S. Pat. No. 5,240,781, issued to Obata et al., discloses an ink ribbon for thermal transfer printers having a thermal transfer layer comprising a wax-like substance as a main component and a thermoplastic adhesive layer having a film forming property.
There are some limitations on the applications for thermal transfer printing. For example, the properties of the thermal transfer formulation which permit transfer from a carrier to a receiving substrate can place limitations on the permanency of the printed matter. Printed matter from conventional processes can smear or smudge, especially when subjected to a subsequent sorting operation. Additionally, where the surface of a receiving substrate is subject to scratching, the problem is compounded. This smearing can make character recognition such as optical character recognition or magnetic ink character recognition difficult and sometimes impossible. In extreme cases, smearing can make it difficult to read bar codes.
Many attempts have been made to provide high integrity thermal transfer printing which is resistant to scratching and smearing, some of which are described above. For example, it is generally known to those skilled in the art that resin binders and/or waxes with higher melting points can provide a higher degree of scratch and smear resistance. However, higher print head energies are necessary to achieve the desired flow to promote transfer and adhesion to a receiving substrate. In U.S. Patent Nos. 5,128,308 and 5,248,652 Talvalkar provides print with improved smear resistance without the need for higher print head energies by employing a thermal transfer formulation which contains thermally reactive phenolic resins and Leuco dyes. These reactive components are said to provide higher intensity print with improved resistance to scratch and smear. The reaction apparently immobilizes the dye. There is no indication the melting point or molecular weight of the resin binder are significantly affected.
Thermal transfer ribbons and reactive components that polymerize to provide scratch/smear/environmental resistant images have lead to stability problems. High molecular weight components have been used to minimize these stability problems but due to their slow reaction rate, a baking step after transfer is required to obtain scratch/smear/environmental resistant images immediately after printing.
There is a continuing effort to provide alternative thermal transfer media which can form printed images with high scratch and smear resistance using relatively low print head energies.